A reference to third party dll is not copied to bin folder even when "Copy Local" is set to True.
Make sure project file has following tag in it
<Private>True</Private>
If this does not help try adding a "using" or "import" in your code as dynamically loaded reference could cause this issue.
It seems if you are not using the library directly in the code Visual Studio detects that its not used and does not include in the output.
